Pneumatic tires for passenger vehicles are conventionally manufactured using a wide variety of types of rubber, including natural rubber. In this regard, natural rubber may be incorporated into various tire components including the carcass and plies, the inner liner, the barrier layer, the sidewalls, the tire crown, and the tread. During the manufacturing process, some tire components including green (i.e., uncured) rubber may be pre-cured by irradiating the outermost layer of rubber with an electron beam, which strengthens the components by cross-linking the polymer chains of rubber molecules in the outermost layer. For example, pre-curing the barrier layer that separates the inner liner from the plies of the carcass provides additional green strength in the area where the barrier layer contacts the plies when the tire is assembled. Consequently, as the tire is cured a more consistent gage of the barrier layer measured between the plies and the inner liner is possible. In another example, pre-curing the rubber material surrounding a calendared ply stabilizes the plies as they are wound into the tire carcass. As a result, less overall rubber material (e.g., up to 20% less rubber) is required to manufacture a pneumatic tire. In this way, electron-beam pre-curing of the surface affords significant advantages by improving the quality and weight of the tire.
Although the pre-curing process allows a tire to be manufactured with less rubber material, there is always some pre-cured rubber stock that is not used at the beginning or the end of an extrusion or calendaring process. While unused rubber stock is typically recycled into other tires, pre-cured rubber stock presents significant problems when reworking the pre-cured rubber stock back into compounds for forming tire components. The pre-cured or cross-linked domains in such compounds tend to produce an area of high stress concentration in a cured tire. Additionally, this stock contains curatives and accelerators so that any reprocessing imparts work to the stock which can lead to domains of pre-vulcanized lumps in the stock. This renders the stock unusable in tire manufacturing.
The problems with reworking pre-cured rubber stock are exacerbated in certain applications such as radial medium truck tire construction, in which the pre-cured rubber stock includes a laminate including a layer of natural rubber (with a partially cured surface layer) and a layer of butyl rubber. In these applications, the cross-linked domains in the natural rubber are again difficult to rework, and furthermore, the butyl rubber is incompatible with other types of rubber. Consequently, it is difficult to produce tire components from non-butyl rubber compounds when reworked pre-cured rubber stock is included in the non-butyl rubber compounds. As a result, the pre-cured rubber stock is largely disposed by selling the pre-cured rubber stock to a third-party vendor in non-tire applications rather than being recycled by reworking the rubber material back into a component for manufacturing tires.
In other applications, unused rubber stock may include a thin protective film adhesively applied to one side of the rubber stock. This film is not rubber material and cannot be reworked into rubber compounds, but the film is typically difficult to remove from the unused rubber stock. Unused rubber stock may also be contaminated along one side with various contaminants. In these circumstances, the entire unused rubber stock is typically disposed by selling to a third party rather than recycling the unused rubber stock into rubber compounds used in producing tire components.
There is a need, therefore, for an improved method for recovering uncured or uncontaminated rubber from unused rubber stock that addresses these and other issues associated with manufacturing tires with recycled components.